Showing papers on "Ion published in 1996"

Structural Information from Ion Mobility Measurements: Effects of the Long-Range Potential

Abstract: In a number of recent studies, information about the structure of large polyatomic ions has been deduced from gas phase ion mobility measurements by comparing mobilities measured in helium to those estimated for assumed geometries using a hard sphere projection approximation. To examine the validity of this approach, we have compared mobilities calculated using the hard sphere projection approximation for a range of fullerenes (C20−C240) to those determined from trajectory calculations with a more realistic He−fullerene potential. The He−fullerene potential we have employed, a sum of two-body 6-12 interactions plus a sum of ion-induced dipole interactions, was calibrated using the measured mobility of C60+ in helium over an 80−380 K temperature range. For the systems studied, the long-range interactions between the ion and buffer gas have a small, less than 10%, effect on the calculated mobility at room temperature. However, the effects are not insignificant, and in many cases it will be necessary to cons...

Generation of nonclassical motional states of a trapped atom.

Abstract: We report the creation of thermal, Fock, coherent, and squeezed states of motion of a harmonically bound {sup 9}Be{sup +} ion. The last three states are coherently prepared from an ion which has been initially laser cooled to the zero point of motion. The ion is trapped in the regime where the coupling between its motional and internal states, due to applied (classical) radiation, can be described by a Jaynes-Cummings-type interaction. With this coupling, the evolution of the internal atomic state provides a signature of the number state distribution of the motion. {copyright} {ital 1996 The American Physical Society.}

Ion-Solid Interactions: Fundamentals and Applications

Abstract: 1. General features and fundamental concepts 2. Interatomic potentials 3. Dynamics of binary elastic collisions 4. Cross-section 5. Ion stopping 6. Ion range and range distribution 7. Radiation damage and spikes 8. Ion-solid simulations and irradiation enhanced transport 9. Sputtering 10. Order-disorder under irradiation and ion implantation metallurgy 11. Ion beam mixing 12. Phase transformations 13. Ion beam assisted deposition 14. Applications of ion beam processing techniques 15. Ion beam system features Appendices: A. Crystallography B. Table of contents C. Density of states D. Derivation of the Thomas-Fermi differential equations E. Centre-of-mass and laboratory scattering angles F. Miedema's semi-empirical model for the enthalpy of formation in the liquid and solid state G. Implantation metallurgy - study of equilibrium alloys.

Free Energy of Ionic Hydration

Abstract: The hydration free energies of ions exhibit an approximately quadratic dependence on the ionic charge, as predicted by the Born model. We analyze this behavior using second-order perturbation theory. The average and the fluctuation of the electrostatic potential at charge sites appear as the first coefficients in a Taylor expansion of the free energy of charging. Combining the data from different charge states (e.g., charged and uncharged) allows calculation of free-energy profiles as a function of the ionic charge. The first two Taylor coefficients of the free-energy profiles can be computed accurately from equilibrium simulations, but they are affected by a strong system-size dependence. We apply corrections for these finite-size effects by using Ewald lattice summation and adding the self-interactions consistently. An analogous procedure is used for the reaction-field electrostatics. Results are presented for a model ion with methane-like Lennard-Jones parameters in simple point charge water. We find t...

Quantum Reservoir Engineering with Laser Cooled Trapped Ions.

Abstract: We show how to design different couplings between a single ion trapped in a harmonic potential and an environment. The coupling is due to the absorption of a laser photon and subsequent spontaneous emission. The variation of the laser frequencies and intensities allows one to ``engineer'' the coupling and select the master equation describing the motion of the ion.

Electron Cyclotron Resonance Ion Sources and ECR Plasmas

Abstract: Contents. Part 1: General Aspects of Electron Cyclotron Resonance Plasmas. Part 2: Specific Aspects of Electron Cyclotron Resonance Ion Sources. Part 3: Electron Cyclotron Resonance Ion Sources for Single or Low Charged Ions. Part 4: Electron Cyclotron Resonance Ion Sources for Multiply Charged Ions. Conclusions. Appendices. Index.

Molecular Dynamics Simulation of Ion Mobility. 2. Alkali Metal and Halide Ions Using the SPC/E Model for Water at 25 °C†

Abstract: We present results of computer simulations of the mobilities of the alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) and the halides (F-, Cl-, Br-, and I-) at 25 °C using the SPC/E model for water and ion−water parameters fitted to the binding energies of small clusters of ions. A simple truncation of the ion−water and water−water potentials was used, and the mobilities calculated from the mean square displacement and the velocity autocorrelation functions, respectively, were found to be in good agreement with each other. The calculations demonstrate, for the first time, cation and anion mobilities that fall on separate curves, as functions of ion size, with distinct maxima. This is in complete accord with experimental trends observed in water at 25 °C. The cation mobilities are also in better agreement with the measured values than the calculations done earlier (J. Chem. Phys. 1994, 101, 6964) using the TIP4P model. The mobilities of the halides calculated here for the SPC/E model are however slightly lowe...

Correlation between the Vibrational Frequencies of the Carboxylate Group and the Types of Its Coordination to a Metal Ion: An ab Initio Molecular Orbital Study

Abstract: The structures and vibrational frequencies of the acetate ion interacting with a metal ion (Na+, Mg2+, and Ca2+) in the unidentate, bidentate, bridging, and pseudobridging forms are studied by ab initio molecular orbital calculations. Effects of a water molecule coordinating to either the acetate ion or the metal ion are also examined. The calculations are carried out by using the self-consistent reaction field method at the Hartree−Fock level with the 6-31+G** basis set. For the species interacting with a divalent metal cation, the lengths of the two CO bonds of the acetate ion are nearly equal in the bidentate form but are significantly different in the unidentate form. The frequency of the COO- antisymmetric stretch of the unidentate species is higher than that of the ionic species, which is in turn higher than that of the bidentate species. The reverse is the case for the COO- symmetric stretch. As a result, the frequency separations (Δνa-s) between the COO- antisymmetric and symmetric stretches for t...

Gas-Phase Conformation of Biological Molecules: Bradykinin

Abstract: Several cationized forms of bradykinin (BK) were generated in the gas phase using matrix assisted laser desorption ionization (MALDI). Accurate collision cross sections were obtained using the ion ...

Cross-Sections, Rate Constants and Transport Coefficients in Silane Plasma Chemistry

Abstract: This paper presents a critical review of the basic data concerning the physics and chemistry of low pressure SiH 4 glow discharges used to deposit hydrogenated amorphous silicon films (a-Si:H). Starting with an updated table of thermochemical data, we analyze the gas-phase elementary processes consisting of i) electron-molecule collisions, ii) ion-molecule collisions, iii) neutral-neutral collisions, iv) other electron and ion collisions involving electron-ion and ion-ion recombination, electron attachment on radicals and detachment of anions, and v) cluster growth kinetics in dusty plasmas. Experimental data or theoretical estimates are given and discussed in terms of cross-sections. collision and reaction rate constants, and transport coefficients. We also analyze the surface processes and reaction probabilities of ions, radicals and molecules.

Electron beam instabilities as generation mechanism of electrostatic solitary waves in the magnetotail

Abstract: We present computer experiments of electrostatic solitary waves (ESW) observed by Geotail in the magnetotail. ESW correspond to broadband electrostatic noise, and they are excited through electron two-stream instabilities along a static magnetic field. We performed one-dimensional electrostatic particle simulations involving two electron beams and an ion beam traveling along the static magnetic field. We vary the density ratio of the electron beams and the thermal velocities of the electron and ion beams. The values of these parameters strongly affect diffusion processes of the electron beams, and accordingly, different types of electrostatic waves are generated. We studied four different cases: cold bistream instability, weak-beam instability, bump-on-tail instability, and warm bistream instability. For these electron beam instabilities, we performed two different runs with cold and hot ions, respectively. The cold bistream instability gives ESW for hot ions and ion acoustic waves for cold ions. The weak-beam instability gives Langmuir waves, while the bump-on-tail instability gives ESW. The amplitudes of the waves excited by the weak-beam and bump-on-tail instabilities are small and do not induce nonlinear decay to ion acoustic waves even in the presence of cold ions. The warm bistream instability gives electron hole modes regardless of the value of the ion temperature. The electron hole mode is a normal mode in the presence of a two-hump electron distribution, and it is regarded as narrowband electrostatic noise. It also leads to formation of ESW, if the phase velocity of the electron hole mode is much larger than the ion drift velocity. A necessary condition for ESW formation through the bump-on-tail instability is derived theoretically, and its significance to Geotail observations is discussed.

Tandem mass spectrometry of large biomolecule ions by blackbody infrared radiative dissociation.

Abstract: A new method for the dissociation of large ions formed by electrospray ionization is demonstrated. Ions trapped in a Fourier transform mass spectrometer at pressures below 10-8 Torr are dissociated by elevating the vacuum chamber to temperatures up to 215 °C. Rate constants for dissociation are measured and found to be independent of pressure below 10-7 Torr. This indicates that the ions are activated by absorption of blackbody radiation emitted from the chamber walls. Dissociation efficiencies as high as 100% are obtained. There is no apparent mass limit to this method; ions as large as ubiquitin (8.6 kDa) are readily dissociated. Thermally stable ions, such as melittin 3+ (2.8 kDa), did not dissociate at temperatures up to 200 °C. This method is highly selective for low-energy fragmentation, from which limited sequence information can be obtained. From the temperature dependence of the dissociation rate constants, Arrhenius activation energies in the low-pressure limit are obtained. The lowest energy di...

Organometallic ion chemistry

Abstract: Preface. 1. Gas-Phase Thermochemistry of Transition Metal Ligand Systems: Reassessment of Values and Periodic Trends P.B. Armentrout, B.L. Kickel. 2. Ab initio Calculations Applied to Problems in Metal Ion Chemistry C.W. Bauschlicher Jr., S.R. Langhoff, H. Partridge. 3. The Electronic Structure of the Neutral, Mono and Di Positive Transition Metal Nitrides, ScN, TiN, VN, and CrN J.F. Harrison, K. La Kunze. 4. Fundamental Studies of Functionalized Hydrocarbons with Transition Metal Ions K. Eller. 5. Electronic State Effects in Sigma Bond Activation by First Row Transition Metal Ions: the Ion Chromatography Technique P.A.M. Van Koppen, P.R. Kemper, M.T. Bowers. 6. Development of a Fourier Transform Ion Cyclotron Resonance (FTICR) Mass Spectrometry Method for Studies of Metal Ion Excited States D.H. Russell, J.V.B. Oriedo, T. Solouki. 7. Gas Phase Organometallic Ion Photochemistry I.B. Surjasasmita, Y.A. Ranasinghe, B.S. Freiser. 8. Applications of Gas-Phase Electron-Transfer Equilibria in Organometallic Redox Thermochemistry D.E. Richardson. Table of Bond Energies.

Effects of Polarizability on the Hydration of the Chloride Ion

Abstract: Polarizable and nonpolarizable potential models for both water and chloride are used to address the issue of surface vs interior solvation of the chloride ion in Cl(H2O)n- clusters, for n up to 255. We find that, even for the largest clusters, simulations with polarizable water models show that the chloride ion is preferentially solvated near the surface of the cluster. This behavior is not observed with a nonpolarizable model. The many-body effects are not directly responsible for this solvation behavior; polarizability appears to be important primarily for its role in facilitating a larger average dipole moment on the water model. Polarizability on the chloride ion is not found to have a substantial effect on the structure of the clusters.

Collisional losses of ring current ions

Abstract: The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce-averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce-averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time-dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low-energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low-energy ion precipitation inside the plasmasphere.

Corpuscular diagnostics and processing methods applied in investigations of laser-produced plasma as a source of highly ionized ions

Abstract: This paper presents a set of complementary corpuscular diagnostics applied in experiments for investigation of laser-produced plasma as a source of ions. The measuring possibilities and methods for processing experimental data of a cylindrical electrostatic ion energy analyzer, a Thomson parabola ion analyzer, various types of electrostatic probes, a detector of neutral atom fluxes, as well as methods for visualization of ion emission areas are discussed. Special attention was focused on the ion-induced secondary electron emission problem and its influence on the accuracy of the measurements.

Dipole bound, nucleic acid base anions studied via negative ion photoelectron spectroscopy

Abstract: The anions of the nucleic acid bases, uracil and thymine, were studied by negative ion photoelectron spectroscopy. Both monomer anions exhibit spectroscopic signatures that are indicative of dipole bound excess electrons. The adiabatic electron affinities of these molecules were found to be 93±7 meV for uracil and 69±7 meV for thymine. No conventional (valence) anions of these molecules were observed.

Molecular Dynamics Study of the Structure and Dynamics of the Hydration Shell of Alkaline and Alkaline-Earth Metal Cations

Abstract: Molecular dynamics simulations of hydrated alkaline and alkaline-earth metal cations at room temperature (T = 300 K) were carried out using the CHARMM22 force field. Dynamic and static properties of systems containing one ion and 123 or 525 water molecules were investigated by analysis of trajectories of 1 ns duration and compared to experimental and theoretical results. In addition, the size and the direction of the elementary motions of both the ions and the water molecules were investigated on the scale of the integration time step of 1 fs. Comparison between systems of different size revealed that for the larger system the diffusion coefficient and the number of hydrogen bonds were increased. Radial pair distribution functions and coordination numbers are in good agreement with X-ray and neutron scattering data. The diffusion coefficient D of bulk TIP3P water in a system with 528 water molecules was by one-fourth higher than the experimental value. Minor differences of approximately 10% between experi...

Coordination of Alkali Metals to Oligosaccharides Dictates Fragmentation Behavior in Matrix Assisted Laser Desorption Ionization/Fourier Transform Mass Spectrometry

Abstract: A correlation is found between the fragment ion yield and the degree of branching for oligosaccharides analyzed by matrix assisted laser desorption ionization/Fourier transform mass spectrometry (MALDI/FTMS). The most branched oligosaccharides produced the least amount of fragment ion. Another relationship is found between the size of the alkali metal ion and the yields of fragment ions. The smallest alkali metal ion produced the greatest amount of fragment ions. The presence of the quasimolecular ion is dependent on both the size of the metal ion and the size of the sugar. A minimum number of saccharide residues are necessary to stabilize the complex and produce a quasimolecular ion. Molecular mechanics calculations were performed and structures obtained consistent with the observed experimental behavior. The site of coordination plays a dominant role in the yield and types of fragment ions observed.

Mobility and solvation of ions in channels

Abstract: In this paper we present some results from a simulation study of the mobility and solvation of ions and uncharged molecules in aqueous solution in smooth cylindrical channels at room temperature This ideal system provides a reference system with which to compare the behavior of water and ions in real porous materials such as zeolites, bucky tubes, and biological channels We find that in channels of radii between 25 and 55 A the water molecules form a cylindrical solvation shell inside the channel walls with some evidence of a second shell in the center of the largest channel Not all protons are involved in hydrogen bonding and a number point toward the walls We attribute this to the concavity of the surface When a sodium ion is added it tends to lie in the center of the channel where it can form the most complete solvation shell Its diffusion rate decreases in smaller channels until it moves too slowly in a channel of 2 A radius to be detected in our simulations This decrease is only partly due to an increase in the mean square force on the ion A range of ions of different sizes were studied in a channel with radius 3 A While the smaller of these ions (F−, Na+, and Ca++) lie preferentially in the center of the channel, larger ions (Cl−, I−, and Cs+) penetrate some way into the layer of water inside the wall and methane and ions with the charges turned off move next to the wall Landau free energy analysis shows that this change is due to the balance between entropy and energy The behavior in smooth channels is quite the opposite of what has been observed in experimental studies and simulations of Gramicidin (pore radius of 2 A), where Cs+ lies closer to the center of the channel and Na+ lies off the axis This difference can be attributed to the specific molecular structure of the gramicidin pores (eg, the presence of carbonyl groups) As in bulk solutions, the mobilities of the ions in smooth channels increase to a maximum with ion size and decrease with increasing magnitude of the charge on the ion, while uncharged species diffuse much more rapidly and show a monotonic decrease with size This behavior is related to the characteristics of the fluctuations of the forces on the solute molecules

Ion-beam processing of silicon at keV energies: A molecular-dynamics study.

Abstract: We discuss molecular-dynamics simulations of ion damage in silicon, with emphasis on the effects of ion mass and energy. We employ the Stillinger-Weber potential for silicon, suitably modified to account for high-energy collisions between dopant-silicon and silicon-silicon pairs. The computational cells contain up to ${10}^{6}$ atoms and these are bombarded by B and As atoms at incident energies from 1 keV up to 15 keV. We show that the displacement cascade results in the production of amorphous pockets as well as isolated point defects and small clusters with populations which have a strong dependence on ion mass and a weaker relationship to the ion energy. We show that the total number of displaced atoms agrees with the predictions of binary collision calculations for low-mass ions, but is a factor of 2 larger for heavy-ion masses. We compare the simulations to experiments and show that our results provide a clear and consistent physical picture of damage production in silicon under ion bombardment. We studied the stability of the damage produced by heavy ions at different temperatures and the nature of the recrystallization mechanism. The inhomogeneous nature of the damage makes the characterization of the process through a single activation energy very difficult. An effective activation energy is found depending on the pocket size. We discuss our results considering the Spaepen-Turnbull recrystallization model for an amorphous-crystalline planar interface. \textcopyright{} 1996 The American Physical Society.

Ion/Ion Reactions in the Gas Phase: Proton Transfer Reactions Involving Multiply-Charged Proteins

Abstract: Multiply-charged cations derived from electrospray of bovine ubiquitin and horse skeletal muscle apomyoglobin have been subjected to reactions with anions derived from glow discharge ionization of ...

Ion acceleration and direct ion heating in three-component magnetic reconnection

Abstract: Ion acceleration and direct ion heating in magnetic reconnection are experimentally observed during counterhelicity merging of two plasma toroids. Plasma ions are accelerated up to order of the Alfen speed through contraction of the reconnected field-lines with three-components. The large increase in ion thermal energy (from 10 eV up to 200 eV) is attributed to the direct conversion of the magnetic energy into the unmagnetized ion population. This observation is consistent with the magnetohydrodynamic and macro-particle simulations.

Proton Transfer Reactivity of Large Multiply Charged Ions

Abstract: Charge-charge interactions dramatically influence the dissociation and proton transfer reactivity of large multiply protonated ions. In combination with tandem mass spectrometry, proton transfer reactions have been used to determine the charge state of an ion and to increase the effective mass resolution of electrospray ionization mass spectra. A model for the proton transfer reactivity of multiply protonated ions, in which protons are assigned to specific sites in an ion based on the intrinsic reactivity of the site and the sum of point-charge Coulomb interactions between charges, is discussed. In combination with experimentally measured rates of proton transfer to bases of known gas-phase basicity, information about the intramolecular electrostatic interactions, gas-phase ion conformation and maximum charge state of an ion produced by electrospray ionization can be obtained.

On the binding of electrons to nitromethane: Dipole and valence bound anions

Abstract: ~Received 28 December 1995; accepted 9 May 1996! Conventional ~valence! and dipole-bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe( nf ) Rydberg atoms with CH 3 NO 2 exhibit a pronounced maximum at an effective quantum number of n*’1361 which is characteristic of the formation of dipole-bound anions @m~CH 3 NO 2 !53.46 D#. However, the breadth ~Dn’5, FWHM! of the n-dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ‘‘doorway’’ dipole anion state. Studies of the electric field detachment of CH 3NO2 formed through the Xe( nf ) reactions at various n values provide further evidence for the formation of both a dipole-bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 1263 meV. Photodetachment of CH3NO2 and CD3NO2 formed via a supersonic expansion nozzle ion source produces a photoelectron spectrum with a long vibrational progression indicative of a conventional ~valence bound! anion with a substantial difference in the equilibrium structure of the anion and its corresponding neutral. Assignment of the origin ~v850, v950! transitions in the photoelectron spectra of CH3NO2 and CD3NO2 yields adiabatic electron affinities of 0.2660.08 and 0.2460.08 eV, respectively. © 1996 American Institute of Physics.@S0021-9606~96!02831-0#